Ionization vacuum gauges may be used to measure low pressures of gases. Ionization vacuum gauges sense pressure indirectly by measuring the electrical ions produced when a gas is bombarded with electrons.
Ionization vacuum gauges may include the following types: (1) thermionic emission vacuum gauges, also called hot cathode vacuum gauges; and (2) field emission vacuum gauges, also called cold cathode vacuum gauges. In hot cathode vacuum gauges, an electrically-heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current flowing through the electrode depends on the number of ions, which depends on the gas pressure in the gauge. The principle behind cold cathode vacuum gauges is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge.
A typical example of a hot cathode vacuum gauge is the Bayard-Alpert vacuum gauge disclosed in U.S. Pat. No. 2,605,431, issued to Bayard on Jul. 29, 1952, which is incorporated herein by reference in its entirety.
The Bayard-Alpert type ionization vacuum gauge is the most common non-magnetic gauge for measuring very low pressures and has been widely used worldwide. The Bayard-Alpert vacuum gauge ionizes the gas molecules within the gauge volume, collects those ions on a thin ion collector wire and measures the resulting current in the ion collector wire. The number of gas molecules present in the volume, or the pressure in the volume, may be determined based on the current measurement.
The Bayard-Alpert gauge may include at least one heated filament that emits electrons toward an anode, for example a cylindrical wire grid. At least one ion collector electrode is placed within and along the axis of the cylindrical wire grid. The negative electrons, which are emitted from the heated filament (a cathode), are accelerated toward the positively-charged cylindrical wire grid (an anode). Electrons pass into the volume of space enclosed by the cylindrical wire grid. In this volume, the electrons collide with any gas molecules in the vacuum chamber and, thereby, produce positive ions. These positive ions are collected by the ion collector electrode disposed within the cylindrical wire grid.
The ion collector electrode may be at nearly ground potential and may be negative compared to the cylindrical wire grid. At constant filament-to-grid voltage and electron emission current, the rate that the positive ions are formed is directly proportional to the density of molecules (pressure) in the vacuum gauge for pressures below about 1×10−3 Torr. The strength of the current may be indicated on a meter, which may be calibrated in units of pressure.
Vacuum chambers requiring very low pressures are used in many different processes. For example, some processes performed in a vacuum chamber produce metallic evaporates. These processes, however, contaminate the ion gauge which is used to measure the pressure in the chamber.
The present invention provides a method and apparatus for reducing the contaminants reaching the ion gauge collector. By directing the contaminants away from the ion gauge collector, as will be described, the ion gauge has less of a likelihood for being damaged, is more accurate, and has a longer maintenance-free life cycle, and is more accurate.